1. Field of the Invention
This invention relates to a gas laser oscillator which generates an AC discharge by a high-frequency electric field in a space filled with a laser medium, thereby generating laser beams.
2. Description of the Prior Art
A gas serving as a laser medium is circulated by a blower through a radiator in an enclosed receptacle in gas laser resonators of the above-described type. A pair of electrodes are disposed so that a high-frequency electric field is established in a portion of a flow pass along which the gas is circulated. When a high-frequency voltage is applied across the electrodes, the gas is excited by the high-frequency electric field established between the electrodes such that laser beams are generated in a direction perpendicular to that of the high-frequency electric field.
There are the following problems to be solved in the above-described gas laser oscillator. That is, the physical structure of the gas discharge caused by the electric field mainly consists of a positive column and an ion sheath both distributed vertically between the electrodes. Electric power in the positive column contributes to the laser excitation but an electric power in the ion sheath does not contribute to the laser excitation. When an AC discharge frequency or a frequency of the high-frequency voltage applied across the electrodes is high, a current flowing in the ion sheath is mainly composed of a displacement current. In this case, an electric power loss is small. When the AC discharge frequency is low, however, the current flowing in the ion sheath contains a conduction current in addition to the displacement current, which results in a loss of the electric power. Consequently, an amount of the conduction current component in the electric current flowing in the ion sheath is increased as the frequency of the AC discharge current is lowered, which results in the increase in the electric power loss. As a ratio of the electric power loss in the ion sheath to the electric power consumed in the positive column is increased, an amount of consumed electric power not contributing to the laser excitation is relatively increased and accordingly, an efficiency of laser oscillation is reduced.
Conventionally, the following two countermeasures have been employed for restraining the power loss in the ion sheath: (1) increasing the positive column voltage and more specifically, increasing the pressure applied to the laser gas and (2) increasing the AC discharge frequency so that the power loss is reduced in the ion sheath. In the case of countermeasure (1), a small-signal gain which is one of fundamental parameters for the laser excitation is lowered and accordingly, the reflectance of an output mirror composing an optical oscillator needs to be increased. However, when the reflectance of the output mirror is increased, the quality of the laser beam is reduced by thermal deformation of the lens.
On the other hand, in the countermeasure (2), the AC discharge frequency needs to be increased to a high frequency above several mega Hertz. Generally, an amplifier system is employed in the case of the power supply at the frequency of several mega Hertz or above. Accordingly, the power supply efficiency is 70% even in the case of an amplifier system of the class C. However, a switching power supply system can be employed when the AC discharge frequency is reduced below several mega Hertz. In this case, the power supply efficiency of 90% can be achieved.